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re: winter humidification wastes energy
11 feb 2005
>ok nick, care to elaborate on what your theory is?

sure. here's the email i sent to lennox... 

sent: monday, january 17, 2005 8:58 am

subject: attn: president/legal--winter humidification wastes energy


i suspect that winter humidification wastes vs saves heating energy, and
the savings claim is an energy myth. people tend to forget that evaporating
water takes heat energy, and that heat energy has to come from somewhere,
even if something like a humidifier belt motor uses little energy by itself.

the heat saved by turning a thermostat down appears to be far less than
the extra heat used to evaporate water, in all but extremely tight houses
with little insulation, eg submarines. claims
that 69 f at 35% rh and 72 f at 19% rh are equally comfortable, but the basic
program in the new ashrae 55-2004 comfort standard predicts that 69 f and 35%
rh and 69.7 at 19% rh are equally comfortable (pmv = -0.537, see below.)

if a 2400 ft^2 tight house has 0.5 ach and say, 400 btu/h-f of conductance,
turning the thermostat down from 69.7 to 69 saves (69.7-69)400 = 280 btu/h.

air at 69 f and 100% rh has humidity ratio w = 0.015832 pounds of water per
pound of dry air, so 19% air has wl = 0.00301, and 39% air has wh = 0.00617.
raising 69 f air from 19 to 39% requires evaporating wh-wl = 0.00316 pounds
of water per pound of dry air. dry air weighs about 0.075 lb per cubic foot.

with 0.5x2400x8/60 = 160 cfm or 9600 ft^3/h or 720 pounds per hour of
air leakage, raising the indoor rh from 19 to 39% requires evaporating
720x0.00316 = 2.275 pounds of water per hour, which requires about 2275
btu/h of heat energy, so it looks like humidifying this fairly airtight
house wastes 2275/280 = 8 times more energy than it "saves." and many 
us houses are less airtight, so humidification would waste more energy.

please modify your energy-savings claim.

thank you.

nick pine

10 screen 9:key off
20 clo=1'clothing insulation (clo)
30 met=1.1'metabolic rate (met)
40 wme=0'external work (met)
50 data 69,35,69.74,19
60 for case=1 to 2
70 read tc,rc
80 ta=(tc-32)/1.8'air temp (c)
90 tr=ta'mean radiant temp (c)
100 vel=.1'air velocity
110 rh=rc'relative humidity (%)
120 pa=0'water vapor pressure
130 def fnps(t)=exp(16.6536-4030.183/(ta+235))'sat vapor pressure, kpa
140 if pa=0 then pa=rh*10*fnps(ta)'water vapor pressure, pa
150 icl=.155*clo'clothing resistance (m^2k/w)
160 m=met*58.15'metabolic rate (w/m^2)
170 w=wme*58.15'external work in (w/m^2)
180 mw=m-w'internal heat production
190 if icl<.078 then fcl=1+1.29*icl else fcl=1.05+.645*icl'clothing factor
200 hcf=12.1*sqr(vel)'forced convection conductance
210 taa=ta+273'air temp (k)
220 tra=tr+273'mean radiant temp (k)
230 tcla=taa+(35.5-ta)/(3.5*(6.45*icl+.1))'est clothing temp
240 p1=icl*fcl:p2=p1*3.96:p3=p1*100:p4=p1*taa'intermediate values
250 p5=308.7-.028*mw+p2*(tra/100)^4
260 xn=tcla/100
270 xf=xn
280 eps=.00015'stop iteration when met
290 xf=(xf+xn)/2'natural convection conductance
300 hcn=2.38*abs(100*xf-taa)^.25
310 if hcf>hcn then hc=hcf else hc=hcn
320 xn=(p5+p4*hc-p2*xf^4)/(100+p3*hc)
330 if abs(xn-xf)>eps goto 290
340 tcl=100*xn-273'clothing surface temp (c)
350 hl1=.00305*(5733-6.99*mw-pa)'heat loss diff through skin
360 if mw>58.15 then hl2=.42*(mw-58.15) else hl2=0'heat loss by sweating
370 hl3=.000017*m*(5867-pa)'latent respiration heat loss
380 hl4=.0014*m*(34-ta)'dry respiration heat loss
390 hl5=3.96*fcl*(xn^4-(tra/100)^4)'heat loss by radiation
400 hl6=fcl*hc*(tcl-ta)'heat loss by convection
410 ts=.303*exp(-.036*m)+.028'thermal sensation transfer coefficient
420 pmv=ts*(mw-hl1-hl2-hl3-hl4-hl5-hl6)'predicted mean vote
430 ppd=100-95*exp(-.03353*pmv^4-.2179*pmv^2)'predicted % dissatisfied
440 print tc,rc,pmv
450 next case

69          35           -.5376486
69.74       19           -.5372599

engineering vp mark hogan said lennox was embarrassed by all this and
he didn't know where their numbers had come from, and he thanked me
for bringing this to their attention and said they are changing their
printed brochures and aprilaire web site energy-savings claim.

this reminds me of david and goliath :-)


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